The invention relates to a mechanical press for example for shaping metal sheets but also for forming massive parts. The press includes a plunger which is movable back and forth by a drive arrangement.
Mechanical presses generally include a main drive with a shaft which is driven by one or several electric motors and which drives one or more eccenters which are directly connected to a plunger by a connecting rod or indirectly by a bell crank. The main shaft is connected to a flywheel which compensates for load variations and which makes the rotation of the main shaft more uniform.
With such presses, the distance-time curve which the plunger follows is determined by the eccenter drive or possibly a bell crank drive. Even if the stroke of the plunger is adjustable the basic distance-time characteristic cannot be changed.
Therefore presses have been proposed wherein the eccenter driving the plunger is driven by a servomotor.
Such an eccenter press is known from DE 41 09 796 C2. Herein, it is proposed to couple an eccenter directly to a servomotor and to connect the plunger to the eccenter by way of a connecting rod. The upper and lower end points of a back and forth movement of the plunger can be variably determined by a flexible control of the servomotor. The servomotor does not rotate uniformly but moves back and forth. Depending on the anglular range of the rotation of the eccenter and depending on the part of the annular movement of the eccenter, the stroke and force generated by the press are different.
In this arrangement, the servomotor needs to generate the force required for the forming of the material at any time. There is no flywheel for releasing stored forces in support of the drive motor.
EP 0 536 804 also discloses a press which is driven by a servomotor but which includes a bell crank drive. The servomotor drives an eccenter by way of a worm drive, the eccenter being connected to the bell drive by way of a connecting rod.
By an appropriate control of the servomotor in either a back and forth operating manner or in a continuous operation, the desired travel distance-time curve for the plunger can be obtained. But also, in this case, the maximum force to be provided by the plunger must be fully supplied by the drive, particularly the drive connecting the servomotor to the eccenter. The otherwise equalizing effect of the flywheel which reduces the load on the drive components disposed between the drive motor and the eccenter is not present. Consequently, the load on the teeth of the worm drive is very high. This is particularly apparent when such a press is to be designed for high press forces.
It is the object of the present invention to provide a motor-driven press which is simple and sturdy in its design and which has a relatively high loading capacity.